Optical retarders are used to alter the relative phase of polarized light passing therethrough, and thus, are well suited for use in applications where control over the polarization is required. In addition to inducing ¼- and ½-wave retardations to control the polarization of light, optical retarders are also used to provide polarization compensation for other optical components in a system. For example, optical retarder compensators are used to introduce a phase delay in incident light to correct for phase differences between two components of polarized light introduced by other optical components in a system.
One particularly important application of optical retarders is providing polarization compensation for liquid crystal display (LCD) panels, wherein residual birefringence of the liquid crystal cell causes linearly polarized light to become slightly elliptical, and wherein the optical retarder maintains the linear polarization in concert with the birefringence of the liquid crystal cell. These compensators, which are often referred to as trim retarder compensators, have been shown to improve system contrast in numerous LCD systems.
For example, consider the 3-panel WGP-based LCoS microdisplay projection system illustrated in FIG. 1. The microdisplay system includes a light source 5, which for example is a high-pressure discharge lamp, and a light rod 7. The light rod 7 homogenizes the cone of light produced by the light source 5 to ensure a spatially uniform light distribution. Optionally, the light rod 7 is a polarization conversion light pipe (PCLP) for producing linearly polarized light. A first lens 8a passes the light from the light pipe 7 to a first folding mirror 9, which directs the light to a first dichroic filter 10. The dichroic filter 10 separates out the blue light from the remaining light, and directs the blue light via second 8b and third 8c lenses, and second 17 and third 16 folding mirrors to a first LCoS display panel 20a. The remaining light, which is transmitted through the dichroic filter 10, is directed via fourth and fifth lenses 8d and 8e and a fourth folding mirror 11 to a second dichroic filter 12. The second dichroic filter 12 separates the remaining light into green and red light, the former of which is directed to a second LCoS display panel 20b and the latter of which passes to a third LCoS display panel 20c. In this instance, each LCoS display panel 20a, 20b, 20c is a vertically aligned nematic (VAN)-mode microdisplay.
Prior to reaching each LCoS display panel 20a, 20b, and 20c, the incident light first passes through a WGP 15, 14, and 13 and a trim retarder compensator 21a, 21b, and 21c, respectively. Each WGP 15, 14, and 13 is a polarizer/analyser formed from a plurality of parallel micro-wires that transmits light having a polarization orthogonal to the direction of the parallel micro-wires and reflects light having a polarization parallel to the direction of the wires (e.g., if the polarizers are designed to pass horizontal or P-polarized light, as illustrated in FIG. 1, the micro-wires will be perpendicular to the plane of FIG. 1). Each LCoS panel 20a, 20b, and 20c alters the polarization of the linearly polarized incident light pixel-by-pixel and reflects the modulated light back to the corresponding WGP 15, 14, and 13. Since each WGP 15, 14, and 13 is orientated at approximately ±45° with respect to the principal direction of light propagation, in addition to serving as a polarizer/analyzer, each WGP 15, 13 and 14 also serves as a beamsplitter for separating the incoming light from the outgoing light by steering or deflecting the light reflected from the each LCoS panel along an output optical path orthogonal to the incoming optical path. More specifically, each WGP 15, 14, and 13 reflects S-polarized light (e.g., polarized light rotated by 90° by pixels in an ON state) to the X-cube 19. The X-cube 19 aggregates (i.e., converges) the image from each of the three color channels and, via the projection lens 18, projects the final image onto a large screen (not shown). Optionally, each color channel further includes a pre-polarizer (not shown) and/or a clean-up analyzer (not shown), which for example, may include one or more WGPs and/or dichroic sheet polarizers.
The trim retarder compensators 21a, 21b, and 21c (herein simply referred to as trim retarders), are compensating elements used to improve the contrast performance level of the microdisplay system, which is otherwise limited by the residual birefringence of the LCoS panels in the dark (e.g., off) state. In particular, each trim retarder 21a, 21b, and 21c introduces a phase retardance that cancels the retardance resulting from the inherent birefringence of the corresponding LCoS panel. The term ‘retardance’ or ‘retardation’, as used herein, refers to linear retardance magnitude as opposed to circular retardance magnitude, unless stated otherwise. Linear retardance is the difference between two orthogonal indices of refraction times the thickness of the optical element. Linear retardance causes a phase difference between two orthogonal linear polarizations, where one polarization is aligned parallel to the extra-ordinary axis of the linear retarder and the other polarization is aligned parallel to the ordinary axis of the linear retarder. In contrast, circular retardance causes a relative phase difference between right- and left-handed circular polarized light.
Linear retardance may be described as either in-plane or out-of-plane retardance. In-plane retardance, expressed as optical path length difference, refers to the difference between two orthogonal in-plane indices of refraction times the physical thickness of the optical element. Out-of-plane retardance refers to the difference of the index of refraction along the thickness direction (z direction) of the optical element and one in-plane index of refraction (or an average of in-plane indices of refraction), times the physical thickness of the optical element. Normal incidence rays in a cone bundle see only in-plane retardance, whereas off-axis rays including oblique rays (i.e. non-normal but along the principal S- and P-planes) and skew rays (i.e. non-normal and incident away from the principal S- and P-planes) experience both out-of-plane retardance and in-plane retardance. Notably, in-plane retardance is not observed for the trivial case of 90° ray angle in the birefringent medium.
In the absence of trim retarders 21a-c, the P-polarized polarized light that illuminates each microdisplay panel in the dark (off) state is slightly elliptically polarized upon reflection due to the residual birefringence of the LCoS panels 20a-c. When the elliptically polarized light, which contains both a P- and an S-component, is transmitted to the corresponding WGP 15, 14, 13, the S component is reflected to the X-cube thus allowing dark state light leakage onto the large screen and limiting the contrast of the projection system.
The use of trim retarders 21a-c improves the contrast level by providing in-plane retardance that compensates for the retardance resulting from the residual birefringence in the LCoS panels 20a-c. More specifically, the trim retarders 21a-c are oriented such that their slow axes are configured at orthogonal azimuthal alignment to the slow axes of the LCoS panels 20a-c (termed “crossed axes”), while their fast axes are configured at orthogonal azimuthal alignment to the fast axes of the LCoS panels 20a-c. The terms slow axis (SA) and fast axis (FA), as used herein, refer to the two orthogonal birefringent axes when the linear retardance is measured at normal incidence. Notably, the SA and FA locations change with off-axis illumination as well as reversing the SA/FA roles for a negative out-of-plane retardance component at a large angle of incidence.
Since the slow axes of the trim retarders 21a-c and LCoS panels 20a-c are configured at orthogonal azimuthal orientations, the role of the fast/slow axes switches from the trim retarder 21a-c to the LCoS panel 20a-c for normal incidence light. In other words, light having a specific polarization is alternately delayed more then less, or vice-versa, in the trim retarder 21a-c and the LCoS panel 20a-c, respectively. The net effect is zero relative delay for the incoming polarization, and as a result, an unchanged polarization (i.e., the output light is not elliptically polarized). The corresponding WGP 15, 14, 13 and/or optional clean-up polarizer then rejects the output light so that the dark-state panel light leakage does not appear on the screen. Since the trim retarders 21a-c do not alter significantly the throughput of the panel on-state, the resulting sequential contrast (full on/full off) is excellent.
The operating principle of each trim retarder 21a-c is further illustrated in FIG. 2, with reference to the core optics of a single-channel light engine. These core optics include a pre-polarizer 30, a WGP 31, a trim retarder 32, a VAN-mode LCoS panel 33, and a clean-up polarizer (not shown). In operation, unpolarized or partial polarized light output from a prior stage illumination (not shown) is passed through the pre-polarizer 30 to obtain P-polarized light. The light is transmitted through the WGP 31 and its polarization extinction ratio is enhanced. The trim retarder 32 preconditions the incoming P-polarization beam and creates an elliptical output. Ideally, the ellipticity in the polarized light incident onto the LCoS panel 33, which is in a dark (off) state, is undone by the residual panel retardance. The reflected light, after completing a double pass through the VAN-LCoS panel 33 and the trim retarder 32, thus remains P-polarized. The remaining P-polarization component transmitted by the WGP 31 is injected back into the illumination system and is eventually lost.
As discussed above, the trim retarder 32 ideally provides an in-plane retardance that matches the in-plane retardance of the corresponding LCoS panel 33 in the off-state. In practice, however, the in-plane retardance of both the LCoS panel 33 and the trim retarder 32 tends to vary within each component due to manufacturing tolerances in device thickness and material birefringence control, as well as operational drifts (temperature, mechanical stress etc). As a result, to ensure adequate compensation it is common to provide a higher in-plane retardance in the trim retarder 32 than that exhibited by the LCoS panel 33. For example, a trim retarder with an in-plane retardance of 10 nm (at λ=550 nm) is often provided to compensate for a VAN-mode LCoS exhibiting a 2 nm in-plane retardance (at λ=550 nm). As is known to those of skill in the art, this mismatch in in-plane value typically requires offsetting of the optic axis of the trim retarder 32, relative to the nominal crossed axes configuration described above. In other words, the trim retarder is clocked-in by rotating its azimuth orientation away from the crossed-axes configuration.
In addition to providing in-plane retardance, it is common for the trim retarder 32 to also provide out-of-plane retardance to increase the field of view. More specifically, it is common for trim retarders to include both an A-plate compensation component for compensating the in-plane retardance and a −C-plate compensation component, which exhibits negative birefringence, for compensating for out-of plane retardance. These full function A/−C-plate trim retarders optionally also include an O-plate component. An A-plate is a birefringent optical element having its extraordinary axis oriented parallel to the plane of the plate. A C-plate is birefringent optical element having its extraordinary axis oriented perpendicular to the plane of the plate (i.e. parallel to the direction of normally incident light). An O-plate is a birefringent optical element having its extraordinary axis (i.e., its optic axis or c-axis) oriented at an oblique angle with respect to the plane of the plate.
Conventionally, trim retarders have been fabricated out of stretched polymers laminated on anti-reflection (AR) coated glass substrates. More specifically, biaxially stretched polymer films, which provide a higher index of refraction along the direction of force (e.g., in the XY-plane) than along the unstretched z-direction, are used to provide full-function A/−C-plate trim retarders (i.e., in general a negative biaxial foil results). Unfortunately, stretched polymer films are not ideal for many polarization-based projection systems due to a lack of retardance uniformity and environmental reliability issues. For example, with regard to the latter, the high temperature and high light flux environment provided in many microdisplay projection applications tends to make the stretched polymers relax over time, and thus lose birefringence.
To obviate the uniformity and reliability issues, U.S. Pat. Appl. No. 20050128391, which is hereby incorporated by reference, teaches an A-plate trim retarder fabricated by spin-coating liquid crystal polymer (LCP) and linear photo-polymerization (LPP) layers on a transparent substrate. The directors of the LC monomers are aligned by the LPP layer and then cross-linked into a polymer host for solid film-like rigidity and reliability. The lack of a suitable −C-plate element is obviated with the introduction of a dielectric thin film form-birefringent (FB) stack into the AR coating designs (FBAR). While this full-function A/−C-plate retarder, has been shown to enhance the image contrast of VAN-mode LCoS display system from several hundreds to one to several thousands to one, it is desirable to reduce the use of organic layers in high light flux projectors applications.
In fact, the presence of any organic material, including lamination epoxy, is generally a source of reliability concerns. One approach to preparing an all-inorganic trim retarder is to use birefringent crystals. While birefringent crystals are more durable and/or more stable, the cost of growing and polishing the crystal plate may be significant, especially for microdisplays having a diagonal of about one inch or greater. As a result, there has been increasing interest in trim retarders fabricated from inorganic and/or dielectric thin films. For example, in U.S. Prov. Appl. No. 11/591,623 filed Nov. 1, 2006, which is hereby incorporated by reference, Tan et al. disclose a full-function all-dielectric trim retarder wherein A-plate retardance is provided by a transversely-inhomogeneous one-dimensional grating structure, and −C-plate retardance is provided by the axially-inhomogeneous one-dimensional FBAR grating structure discussed above.
Another method of introducing form birefringence in a dielectric material is to use oblique angle deposition, wherein a thin dielectric film is deposited, either by evaporation or sputtering, at an angle, to provide a porous form-birefringent layer. More specifically, and as illustrated in FIG. 3, this technique uses oblique incident vapor flux 40 to effect atomic shadowing 42 on a substrate 44 and provide microstructures with isolated columns of material 46 growing toward the vapour source. The optical properties of the thin film are dependent on the material used, the porosity of the microstructure, and the orientation of the columns. In general, the orientation of the columns is related to the incident vapor flux angle θv (angle of incidence measured from the normal). While the incident vapor flux angle θv may be anywhere between 0° and 90°, the trend is to use high angles of incidence (e.g., greater than 75°) to maximize the tilt of the columnar microstructure.
For example, in U.S. Pat. No. 5,638,197, Gunning et al. discloses an inorganic thin film ‘O-plate’ compensator for improving gray scale performance in twisted nematic (TN) type LCDs. The optimum deposition angle, θv is reported to be 76°.
In U.S. Pat. No. 6,097,460, Shimizu et al. disclose a phase retarder film containing TiO2 that can be utilized for improving the viewing angle characteristics of a TN-type LCD or a super twisted nematic (STN) type LCD. In order to provide the required A-plate retardation values ranging between 20 nm to 200 nm, the incident vapour flux angle ranges from 50° to 85°.
In U.S. Pat. Nos. 5,866,204 and 6,248,422, Robbie and Brett describe a method of obliquely depositing thin films, wherein in a first stage the incident vapor flux angle (angle of incidence measured from the normal) is fixed at a value typically greater than 80° to produce a porous columnar microstructure, and wherein in a second stage the incident vapor flux angle approaches 0° to provide a dense and uniform capping layer.
In U.S. Pat. No. 7,079,209, Nakagawa teaches a retardation compensator for increasing the viewing angle and/or improving the contrast ratio of a TN-type LCD-based projection system. According to one embodiment, the retardation compensator includes an array of rod-like columns formed by oblique angle deposition as described in U.S. Pat. No. 5,638,197 (e.g., with an optimum deposition angle of about 76°).
While these references do provide form-birefringent thin films via oblique angle deposition, the above-described porous microstructures are not ideal for projection systems. For example in many instances, such as when the substrate is not rotated during the deposition process, the layer thickness and hence the retardation uniformity will be poor. In all instances, a high porosity is expected to cause reliability problems. In particular, the highly porous microstructures may delaminate from the supporting substrate in the high temperature and/or high light flux environment provided in polarization-based projection systems, such as cinematic projector. Also, the porous film is expected to allow moisture to infuse into the birefringent layer(s) thereby changing its retardation property with changing humidity environment.
In addition, the large in-plane birefringence provided by the porous microstructures makes these structures particularly unsuitable for VAN-mode reflective LCD systems (e.g., VAN-mode LCoS projections TVs). In general, it is desired that trim retarders for VAN-mode reflective LCoS systems only exhibit small amounts of in-plane birefringence, to reduce back reflections and/or improve angular tuning of contrast optimization. For example, with respect to the latter, trim retarders with a large in-plane retardance (e.g., over 30 nm) typically exhibit an overly sensitive tuning curve (i.e., contrast level versus clocking angle).